1. Field of the Invention
This invention relates to the preparation of natural tissue for implantation.
2. Description of Prior Art
Natural aortic valves taken from animals or humans have been used for a number of years for allo and xenograft replacement of diseased valves. This has experienced only limited success, however, as in a large proportion of cases the replacement valves develop leakage. The incidence of incompetency has run from 40 to 80%. This occurs primarily because the valve is a very flexible and elastic structure which does not maintain its normal physiologic state when removed from the donor heart. This readily distortable element must then be matched with the flexible structure of the patient's heart, which also is not in its normal physiologic state when prepared for the valve replacement. With distortion of the flexible valve being unavoidable, and with no two valves being the same, it becomes virtually impossible to affix the valve to a stent or the patient's annulus at the proper location to maintain the valve's natural configuration. Consequently, undesirable stresses are introduced into the valve. This may result in immediate incompetence of the valve. Even where this does not prevent competence initially, the existence of the stresses in the valve over a period of time frequently causes premature failure.
If an appropriate tanning fluid is applied to a natural heart valve, its tissue becomes fixed so that it loses its characteristic of extreme flexibility and elasticity. As a result, in order to assure retention of approximately normal geometry, the cusps of the allo and xenograft valves have been packed with cotton, and the valves then have been fixed by tanning. This, however, has not solved the problem of valve failure because it never has been possible to impart exactly its natural configuration to a valve by packing it with cotton or similar material. Consequently, there are inevitably variations from the normal valve shape, and these differences become fixed in the valve by the tanning operation. These shape variations introduce stresses in the valve, which may cause it to fail subsequently.
Another problem encountered from such tanning of the valve is that shrinkage occurs as a result of this procedure. The amount of shrinkage is not the same in different valves, so that there is no way of predicting how much shrinkage will take place. Later, when the valve is exposed to biologic fluid after it has been implanted, it experiences some re-expansion. Again, this is an unknown amount. These factors contribute to the difficulty in properly mounting the valve and make the introduction of undesirable stresses practically unavoidable.
Similar problems have been encountered with vessels which also have been prepared for implantation by fixation with a tanning fluid. Problems have included inadequate penetration of the fixative, a lack of uniformity of fixation and insufficient tensile strength in the prepared vessels.